Use smtp.gmail.com as EMAIL_HOST

[Xpirix]

This PR is a fix for the email-sending feature since noreply@qgis.org goes sometimes in the spam folder.

Change summary:

• Use smtp.gmail.com as email host
• Replace noreply@qgis.org with automation@qgis.org

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 82.45%. Comparing base (32e0504) to head (5bfb1ca).

Files	Patch %	Lines
qgis-app/plugins/tests/test_plugin_update.py	66.66%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #356 +/- ## =========================================== + Coverage 36.08% 82.45% +46.37% =========================================== Files 111 149 +38 Lines 4900 5933 +1033 =========================================== + Hits 1768 4892 +3124 + Misses 3132 1041 -2091 ```

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